A compact excimer-dye laser system is developed for subpicosecond pulse generation at
193 nm. In this setup a double-discharge excimer laser (EMG 150) is used both for
pumping the seed-pulse generator and for amplification. The seed-pulse generator is a
distributed-feedback dye-laser-based subpicosecond dye laser system, generating a
400 fs pulse at 537 nm and a broadband nanosecond pulse centered at 690 nm. By
sending these pulses through a H2 Raman cell in a collinear beam, part of the red
pulse is amplified, which coincides with the spectral and temporal gain window of the
Raman amplifier pumped by the transform-limited 537-nm pulse. Phase-matched frequency
mixing of these Raman-amplified pulses with the frequency-doubled 537-nm pulses
results in seed-pulse energies of more than 0.1 jJ at 193 nm, with good spatial,
spectral and amplitude stability. By double-pass amplification of the seed pulses in
the amplifier section of the EMG 150 excimer laser, typically 0.5 mJ output pulses of
-800 fs duration are obtained in a 6 mm diameter circular beam. Additional pulse
compression in a prism compressor resulted in significant decrease of the pulse
duration to less than 400 fs.

Generation and amplification of femtosecond laser pulses is discussed. We discribe
different amplification schemes and present detailed results on the spatial and
temporal properties of the amplified pulses. Pulses of approximately 100 fs and
peak intensities of several times 1O are obtained.

This talk is not a survey of the numerous papers recently published but rather a
brief description of some recent researches carried out by my closed colleagues and
myself. I would like to popularize "electromagnetic optics" by giving an idea of
its methods, its possibilities . . . and also its limits. -

Huge dynamic Stark shifts of atomic energy levels in xenon induced by high-intensity laser light have
been mesured by a new method based upon multiphoton ionization by a tunable femtosecond laser source.
Stark-induced multiphoton resonances appear as identifiable structures in the photoelectron energy spectra1. The
change in the structures positions, when the photon energy is tuned, is directly related to the energy change of
the atomic levels as a function of intensity. Shifts of the order of the electron-volt have thus been measured in
xenon and found to vary linearly with intensity.

In this paper we report the generation of femtosecond pulses in the near infrared by passive
mode locking of a Rhodamine 700 dye laser.Two saturable absorbers were used to produced pulses
from 775 to 800 nm. In a short cavity to avoid a multiple pulses regime, pulses shorter than 50 fsec
have been obtained.

We report on the design and performance of an improved copper vapor laser pumped
amplifier for femtosecond optical pulses. Consideration of the large duration mismatch
between pump and seed pulses, the short energy storage time of the gain dye, spatial
mode matching, and polarization effects led to an amplifier design of 1.5% efficiency,
which is greater than previous designs. Pulses of > 10 tJ energy and -65 fsec duration
are generated that are focussable to less than twice the diffraction limit. The multiple
pass amplifier incorporates optimized mode matching, an antireflection coated flowing dye
cell, and a polarized, spatially smoothed pump beam.

We report the generation of a variety of periodically modulated pulse trains in the
CPM dye laser. These pulses are reminiscent of high-order solitons. We have identified the
nonlinearity which prevents the formation of pulses which are solutions of the nonlinear
Schrodinger's equation. A modification of the laser cavity removes this nonlinearity and
true N = 2 solitons have been generated.

We want to make the best use of the mesoscopic or macroscopic transition dipolemornent
of excitons by controling the size and dimension of the materials. First, the semiconductor
microcrystallites embedded in insulators or glasses are shown to have the mesoscopic transition
dipolemoment of excitons1 . As a result, the exciton in CuC1 microcrystallite with the radius 80
A superradiatively decays in 100 pico-second2. Futhermore, the exciton-exciton interaction and
the decay and relaxation of the exciton work effectively in deviating the excitons in the microcrystallite
from ideal bosons and this system shows the large third-order optical susceptibility"3
of an order of iO esu when we assume the 0.1% packing of the CuC1 microcrystallites. These
predictions have been confirmed expriment ally4'5.
Second, the two-dimensional excitons in quantum wells can superradiatively decay in the
direction perpendicular to the surfaces2 . This brings about the quick response and the deviation
of the excitons from ideal bosons at the same time. The latter effect as well as the
two-dimensional macroscopic enhancement of the exction transition dipolemoment enhance the
third-order optical susceptibility3. Both the superradiative decay rate and increase as does
the coherent length, which is inversely proportional to the square root of the exciton line width6.

We report time-resolved pump-probe measurements of both absorption and refractive
index for GaAs thin films with 80 fs time resolution. The probe photon energies are
near the band-edge and near the initial excited states, and photoexcited carrier densities
are varied from 1017 to 1019 cm3. Our results reveal the instantaneous band gap renormalization
and the importance of carrier-carrier scattering and intervalley scattering.
Mechanisms which cause an ultrafast change in refractive index are also discussed.

We study new structures peaked at the giant two-photon absorption (GTA) of
biexcitons and peaked at h1P in the exciton region which appear in the excitation
spectrum for the emission at h w of the biexciton origin in CuC1 at 2 K in the
excitation photon density of ' 5Mw/cm2. An energy relation, thw1P .
+ ALT is obtained, where b and ALT are the binding energy of a biexciton and
the L-T splitting energy of an exciton, respectively. Existence of a complex of
biexciton-polariton or triexciton is implied as an interpretation of this
phenomenum. Discussion is presented in relation to the broad band at GTA.

Spatial and polarization dependence of the beam diffracted by a transient
phase grating induced in a transparent glass enables, for the first time, evidence of a fifth
order nonlinear process. Discrimination between direct high order phenomena and cascade
contributions due to lower nonlinearities is discussed in view of an anharmonic oscillator
model expanded up to sixth order.

We have used the picosecond optoelectronic switching technique to study carrier
recombination times and mobilities at different conditions in semiconductors.
Recombination times were measured by correlation technique, e.g. linear and
non'inear recombination rates. The photoconductivitv method was also used to
measure the dependence of the carrier mobility on the applied field strength in
GaAs The mobility was determined from the slope of the v(E curve. Using this
technique we were able to distinguish between the carrier mobilities in GaAs in
different valleys of the conduction band at different preparation conditions. The
PS optoelectronic switching technique was also used to measure the response of a
fast Dhotodiode of a 6 8Hz transistor and the dispersion of a ps electrical pulse
1on a cable.

The interest in fast risetime, high power photoconductive devices stems from
several potential applications including pulse, impulse, and high power microwave
generation [1, 2]. As the voltage becomes large and the switching speed becomes
extremely fast, the generation of high power electrical pulses using photoconductive
devices becomes very complicated because of effects such as surface flash-over,
stray inductance, stray capacitance, and abrupt impedance transitions. Two ap-
proaches were taken to produce maximum output voltage amplitude without degradation
in switch risetime. A coaxial transmission line with a switch holder filled with a
high dielectric strength liquid was fabricated for long pulsewidths ( 2 ns) . For
the narrow pulsewidths ( 2 ns) a quasi-radial transmission line was fabricated.
By incorporating a photoconductive bulk GaAs device with opposite gridded elec-
trodes into these specially designed energy storage elements, and illuminating the
switch with a mode-locked Nd:YAG laser, electrical pulses with risetimes of several
picoseconds have been realized.

Femtosecond pump-probe experiments have been performed on asymmetric GaAs/AIGaAs quantum well structures,
using a spectrally narrow excitation beam whose center wavelength could be tuned between 740 and 800 nm. The respective
widths of the wells were chosen to exhibit n=1 and n=2 excitonic absorptions of the wide well clearly separated from the
n'=l narrow well excitonic absorption line, which lies in between the two others.
We monitor the time evolution of the photoexcited carrier population in both wells by measuring the variations of
the sample transmission. This method is another approach to the study of the tunneling transfer between coupled quantum
wells, mostly addressed by use of time-resolved photoluminescence techniques.
The sample recovery after excitation shows drastic changes depending on the pump intensity. At high intensities, all
three transitions are bleached and we observe an important band-filling. At low intensities we see a decrease with time of
the carrier population in the narrow well, along with a simultaneous increase in the wide well. This shows that carriers tunnel
through the barrier with a characteristic time much smaller than the recovery time measured at high intensities, and thus
demonstrates that the presence of a large number of carriers dramatically reduces the tunneling efficiency between the two
wells.

Transient pump-probe reflectivity measurements are performed on crystalline and amorphous
Silicon samples with 50 fs optical pulses at 2 eV. The excited carrier densities range from
1017cm3 up to a few 1021cm3. In both cases the reflectivity signal is dominated by a
Drude-like carrier response. Crystalline Silicon shows a distinct subpicosecond feature due to
the cooling of the optically excited hot carriers with a time constant of 200-300 fs. Diffusion
and Auger-recombination come into play at higher carrier densities. A superlinear increase of
instant reflectivity signal with excitation fluence is due to two-photon absorption (TPA) with
a TPA-coeffiecient f:37+-5 cm/GW. In amorphous Silicon the TPA process is not observable.
The recovery of the induced negative reflectivity changes is dominated by trapping into
bandtail and defect states at lower carrier densities. At higher densities a non-radiative
recombination process dominates the relaxation of free carriers in both materials. Comparison
with crystalline Silicon clearly demonstrates the enhancement of the Auger-recombination
process in disordered materials by more than an order of magnitude.

Time resolved measurements of the hot carrier relaxation in nP have been performed
at room temperature using four different femtosecond techniques. At carrier densities
between 1016cm3 and a few times 1018cm3 carrier-carrier scattering has been found to
dominate the initial relaxation ensuring the internal thermalization of electrons and holes on a
time scale of 1OO'2OO fs. At high excitation densities the subpicosecond cooling of the
electrons is found to be clearly slower than expected from simple calculations of the
e-LO-phonon interaction. The different relaxation behaviour in GaAs is attributed to strong
intervalley scattering mechanisms.

Electron and hole tunneling transfer processes in asymmetric double quantum well structures are investigated
by time-resolved picosecond photoluminescence. Change from nonresonant to resonant tunneling is achieved with a
perpendicular eleitric field. Electron transfer times decrease if the two electron subbands in the two quantum wells
are energetically aligned. Both nonresonant and resonant electron transfer times decrease strongly with the barrier
thickness. The resonant times are more than one order of magnitude smaller than the nonresonant times for the
same barrier thickness. The buildup of delocalized coherent states at resonance would lead to much shorter resonant
transfer times than we observe experimentally. This discrepancy is discussed in terms of state broadening. Hole
transfer times also decrease at specific electric fields showing a resonance feature, which can be attributed either to
the resonant transfer of n=1 heavy holes to an n=1 light hole level or to the onset of the optical phonon scattering
from the n=1 heavy hole level to the neighbouring n=1 heavy hole level.

Keywords/Phrases

Keywords

in

Remove

in

Remove

in

Remove

+ Add another field

Search In:

Proceedings

Volume

Journals +

Volume

Issue

Page

Advanced PhotonicsJournal of Applied Remote SensingJournal of Astronomical Telescopes Instruments and SystemsJournal of Biomedical OpticsJournal of Electronic ImagingJournal of Medical ImagingJournal of Micro/Nanolithography, MEMS, and MOEMSJournal of NanophotonicsJournal of Photonics for EnergyNeurophotonicsOptical EngineeringSPIE Reviews